Apr 21, 2015
Exoplanetary LEECH

Sen—In 1995, astronomers announced the detection of the very first planet found orbiting a star like the Sun. In the years since, a slew of different methods have found nearly two thousand of such planets, ranging in size from smaller than Mercury to ones far heftier than Jupiter.

Their tremendous distance from Earth and proximity to their host stars make these planets extremely difficult to detect directly; that is, getting actual pictures of them. However, a very few have been seen in telescopic images.

HR 8799 is a star about 130 light years from Earth. It’s more massive, hotter, and brighter than the Sun, but most importantly it and its planets are young, about 30 million years old. That’s critical in this case: Young planets are still hot, glowing with the heat leftover from their formation. That makes them glow brightly in infrared light, making them easier to spot. Also, stars like HR 8799 don’t put out nearly as much light in the infrared as they do in visible light (the kind our eyes and most telescopes see), making the planets easier to spot because the contrast is higher, too.

This exoplanet system is back in the news because it was the first target of the new Large Binocular Telescope (LBT) Exozodi Exoplanet Common Hunt, or LEECH. In general, it’s difficult to observe exoplanets even in the infrared, because the telescope and camera themselves are warm, and glow at the very wavelengths the astronomers are trying to observe. But LEECH uses a system that has been optimized to reduce that glow, allowing very precise observations at wavelengths that have been previously inaccessible.

The new observations pinpointed the positions of the four planets orbiting HR 8799, firming up the numbers from previous observations. Using these observations (and making simple assumptions like circular orbits), the researchers found the most likely periods of the planets (the times it takes them to orbit the star) are 456, 228, 114, and 57 years (from the outermost planet seen to the innermost).

That’s interesting! Do you spot the pattern? If you divide those numbers by the shortest period of 57 years, you get 8, 4, 2, and 1.

That’s no coincidence. This is a called a resonance, and can occur naturally in planetary systems. Over millions of years, especially in young systems, the planets interact with each other gravitationally, and their orbits get changed. When the planets get into orbits with simple numerical ratios for their periods (like 2:1), they can strongly interact.

But sometimes this same resonance prevents the planets from interacting. For example, Pluto orbits the Sun twice for every time Neptune goes around twice. This simple resonance (3:2) means that every time Pluto gets near Neptune’s orbit, Neptune is a quarter of the way around its orbit, preventing the two from ever getting near each other.

Resonances therefore can tell you about the stability (or instability) of a planetary system. It also puts limits on the masses of the planets; if they are very low mass then they don’t interact strongly, and the resonances won’t destabilize the system. In the case of HR 8799’s planets, the new observations indicate the system is stable… for now. It doesn’t look like there will be any collisions between the planets for at least several million years.

Not only that, but resonances also help understand possible planets inside the four seen. The innermost planet, HR 8799e, still orbits the star farther out than Saturn orbits the Sun, so there could very well be smaller planets orbiting closer in. A possible fifth planet has been proposed, and these new observations show it could exist, but would have to be less than eight times Jupiter’s mass, and would be close enough to the star that it would be very difficult to detect with current methods.

I find all this rather intoxicating. When I was younger, we didn’t even know if any other stars had planets. Now we know of thousands, we’ve seen them directly, we can study them, and a whole new field of science has grown around them. With surveys like LEECH we’ll be able to study them more easily, and with new techniques developed, and more advanced tech always being created, the field of exoplanets study has a long, rich future ahead of it.